The motive behind an attack on U.S. Ambassador to South Korea Mark Lippert is unknown, the U.S. State Department said on Thursday.

Lippert, 42, underwent two-and-a-half hours of surgery after he was slashed in the face and wrist by a knife-wielding Korean nationalist in Seoul about 7:40 a.m. local time Thursday (2240 GMT Wednesday).

“We cannot speculate on a motive at this time,” the State Department statement said.

The assailant, Kim Ki-jong, 55, was quickly subdued by participants at the breakfast.

Kim reportedly told police he acted alone. When he attacked the ambassador, he reportedly shouted, "South and North Korea should be reunified!”

Slashed with knife

Lippert was attacked as he was about to speak at a breakfast forum to discuss Korean reunification. He was listed in stable condition Thursday at the Sinchon Severance Hospital in Seoul.

Hospital director Jung Nam-shik said Lippert needed 80 stitches to close the deep gash to his face -- about 11 centimeters (four inches) long and three centimeters (one inch) deep, located from his right cheekbone to jaw. Doctors also repaired tendon and nerve damage to his thumb and two fingers on his left hand.

Later Thursday, Lippert later wrote on Twitter that he and his family were deeply moved by an outpouring of support and that he would soon return to work, to advance U.S.-South Korean relations.

Before he became ambassador in Seoul in October 2014, Lippert was a U.S. Senate aide to now-President Barack Obama and served in the U.S. Navy in Afghanistan and Iraq, winning the Bronze Star. He was chief of staff for former Defense Secretary Chuck Hagel before taking up his post in Seoul.

Lippert has become well-known during his short time in Seoul, through his presence on social media and in speeches and presentations during his time in the South Korean capital, the AP reported.

Well wishes for recovery

Ben Rhodes, deputy national security adviser to U.S. President Barack Obama, told MSNBC in an interview Thursday, “I expect he'll be back on his feet and in his job as soon as possible.”

Obama spoke with Lippert after the attack, wishing him a speedy recovery, said White House national security spokeswoman Bernadette Meehan. The

U.S. State Department condemned the attack, but added “The U.S.-ROK alliance is strong; we will not be deterred by senseless acts of violence," spokeswoman Marie Harf said in a statement. U.S. Secretary of State John Kerry, who was in Saudi Arabia for meetings with Gulf state officials, echoed that comment, saying, "The United States of America will never be intimidated or deterred by threats or by anybody who harms any American diplomats."

Democratic Senator Patrick Leahy said in a statement that he was "heartsick ... about the senseless attack on Mark. ... He is as devoted a public servant as they come, a great credit to both of our countries."

Lippert served as a former policy aide to Leahy. South Korean President Park Geun-hye said the incident was "not only a physical attack on the U.S. ambassador in South Korea but also an attack on the Korea-U.S. alliance and we will not tolerate it."

South Korea's Foreign Ministry condemned the attack and vowed a thorough investigation and strengthened protection of embassies.

The South Korean Defense ministry said the attack will not affect the ongoing joint military exercises, which began Monday and involve thousands of troops from the United States and South Korea and are meant to prepare for a possible attack from North Korea. The exercises continue for eight weeks.

'Callous' N. Korea Remarks

The United States called North Korea "outrageously callous" Thursday after the communist nation hailed the bloody attack on the American ambassador to Seoul as "deserved punishment for the warmonger U.S."

The North's official Korean Central News Agency released the remarks in a dispatch hours after a pro-North Korean activist attacked the ambassador.

"We have seen the statement which was outrageously callous, but unfortunately consistent with the nature of the regime and its rhetoric," a State Department spokesperson said in comments sent to Yonhap News Agency.

South Korean media reported that Kim has a long history of ultra-nationalist activism and violence that includes burning a U.S. flag on the embassy grounds in Seoul in 1985, staging one-man protests against Japan over disputed islands, and attempting to light himself on fire during a protest in front of the presidential Blue House in 2007.

Seoul's Foreign Ministry said it was the first time a foreign ambassador stationed in modern South Korea had been injured in a violent attack. But police said Kim was arrested in 2010 for throwing concrete stones at the Japanese ambassador and received a suspended two-year jail sentence.

In a recent blog post believed to have been written by Kim, he blamed the joint U.S.-South Korea military exercises for preventing the reunions of families that have been separated since the end of the Korean War in 1953.

North Korea recently cited the joint drills as the reason why they would not permit the reunions to take place.

The event at which the attack occurred, the Korean Council for Reconciliation and Cooperation, is a coalition of various groups that support the peaceful unification of the two Koreas.

How attack happened

Michael Lammbrau of the Arirang Institute think tank was at the meeting and told Reuters, "The guy comes in. ... He yells something, goes up to the ambassador and slashes him in the face.”

Lammbrau said Kim shouted about Korean independence while he was being restrained.

"It sounded like he was anti-American, anti-imperialist, that kind of stuff," he said.

"The ambassador fought him from his seat. ... There was a trail of blood behind him," Lammbrau told Reuters.

U.S. ambassadors have security details, but the size largely depends on the threat level of the post, the AP reported.

Seoul is not considered to be a particularly high threat post despite its proximity to the North Korean border. It's not clear how many guards Lippert had, but they would have been fewer than the ambassadors in most of the Mideast, the AP reported.

VOA News Producer Youmi Kim contributed to this report. Cindy Saine contributed to this report from Washington. Some material for this report came from Reuters and AP.